The present invention relates to a light emitting element such as laser diode element (LD), a light emitting diode element (LED) and the like comprising a nitride semiconductor (InxAlyGa1xe2x88x92xxe2x88x92yN, 0xe2x89xa6X, 0xe2x89xa6Y, X+Yxe2x89xa61).
Since a multilayered reflective membrane layer formed by alternately depositing two layers with different reflectivities has an extremely high reflectivity, the layer is used in a variety of purposes. Such a multilayered reflective membrane layer is generally formed by depositing pairs of a first layer with a film thickness of xcex/4na (xcex: incident light wavelength, and na: index of refraction) and a second layer with a film thickness of xcex/4nb [xcex: incident light wavelength, and nb: index of refraction (nbxe2x89xa0na)] and in order to obtain a further high reflectivity, the reflectivity difference of the first layer and the second layer is required to be large.
For example, in the case of forming a multilayered reflective membrane using AlaGa1xe2x88x92aN (0 less than a less than 1) for the above first layer and GaN for the second layer, in order to make the reflectivity difference between these layers wide, the Al mixed crystal ratio a of the AlaGa1xe2x88x92aN is required to be high.
As a light emitting element using such a multilayered reflective membrane, applicant of the present invention has developed a short wavelength laser oscillating in a violet to green region as disclosed in Japanese Laid-open Patent Publication No. 2001-7444. A schematic cross-sectional view of the laser element is illustrated in FIG. 7. The laser element 10 of FIG. 7 is a surface emitting laser element and formed by depositing an n-type nitride semiconductor layer, an active layer 6 composed of InxGa1xe2x88x92xN (0 less than x less than 1) and a p-type nitride semiconductor layer in this order on a sapphire substrate 1 through a buffer layer 2. In the laser element 10, the n-type nitride semiconductor layer is composed of an n-type contact layer 3, a second n-type clad layer 4, an n-type multilayered reflective membrane 44, and a first n-type clad layer 5 formed on the buffer layer 2. On the other hand, the p-type nitride semiconductor layer formed on the active layer 6 is composed of a second p-type clad layer 7, a first p-type clad layer 8, and a p-type contact layer 9. Further, a negative electrode is formed on the n-type contact layer 3 and a positive electrode is formed on the p-type contact layer 9.
In such a laser element 10, the multilayered reflective membrane 44 is formed in the n-type nitride semiconductor layer nearer to the substrate 1 side than the active layer 6. The multilayered reflective membrane 44 functions as a mirror (light reflecting) layer and reflects the emitted light from the active layer 6 and enclosed it in the active layer 6. In the laser element 10 of FIG. 7, the multilayered reflective membrane 44 is formed by alternately depositing, for example, each 10 layers of AlaGa1xe2x88x92aN (0 less than a less than 1) and GaN.
In a multilayered reflective membrane comprising AlaGa1xe2x88x92aN and GaN, if the Al mixed crystal ratio a of the AlaGa1xe2x88x92aN layer is increased in order to increase the reflectivity difference between these layers, as a is increased, the crystallinity of the AlaGa1xe2x88x92aN layers is deteriorated. If multilayered reflective membrane with deteriorated crystallinity is formed in a laser element 10, there occurs a problem that the light emitted from an active layer 6 is diffused in the multilayered reflective membrane 44 and the multilayered reflective membrane 44 cannot sufficiently exhibit the function as the reflective membrane to result in increase of the threshold electric current value and the threshold voltage for laser oscillation.
Further, in the laser element 10, if the crystallinity of the multilayered reflective membrane 44 is low, there is a problem that the crystallinity of the respective nitride semiconductor layers to be grown on the multilayered reflective membrane 44 is deteriorated and morphological abnormality takes place and cracks are formed.
On the other hand, if the Al mixed crystal ratio a is lowered in order to suppress the crystallinity deterioration of the AlaGa1xe2x88x92aN layer, the reflectivity difference between the AlaGa1xe2x88x92aN layer and the GaN layer becomes small and the reflectivity of the multilayered reflective membrane is decreased. If the multilayered reflective membrane with a low reflectivity is formed in the laser element 10, the light cannot effectively be enclosed in the active layer 6 to result in difficulty of laser oscillation.
The present invention is to solve the above-described problems and to provide a gallium nitride-based multilayered reflective membrane with an excellent crystallinity while keeping a high reflectivity and a gallium nitride-based light emitting element using such a multilayered reflective membrane.
A multilayered reflective membrane of the present invention includes an AlaGa1xe2x88x92aN layer (0 less than a less than 1) having a thickness of (xcex11xc2x7xcex)/(4n1) (xcex: incident light wavelength, n1: index of refraction) and a GaN layer having a thickness of (xcex12xc2x7xcex)/(4n2) (n2: index of refraction) which are deposited alternately and satisfy the relationship of 0 less than xcex11 less than 1 and xcex11+xcex12=2.
Conventionally, in a multilayered reflective membrane depositing a plurality of pairs of an AlaGa1xe2x88x92aN layer and a GaN layer, the film thickness of the AlaGa1xe2x88x92aN layer and the film thickness of the GaN layer composing one pair are xcex/4n1 (that is, xcex11=1) and xcex/4n2 (that is, xcex12=1), respectively. Whereas, according to the present invention, while keeping xcex11+xcex12=2 as it is before, xcex11 is kept less than 1 to make the film thickness of the Alxcex1Ga1xe2x88x92xcex1N layer thinner than the conventional value xcex/4n1, so that a multilayered reflective membrane with an excellent crystallinity while keeping a high reflectivity can be obtained. Further, since the AlaGa1xe2x88x92aN layer is made thinner than before, even if the Al mixed crystal ratio a is made relatively high, the crystallinity deterioration can be suppressed and a multilayered reflective membrane with a high reflectivity can be obtained.
In such a multilayered reflective membrane, the Al mixed crystal ratio a of the AlaGa1xe2x88x92aN layer is preferable to satisfy 0.2xe2x89xa6axe2x89xa60.8. It is because if a exceeds 0.8, the crystallinity deterioration of the multilayered reflective membrane probably becomes significant, whereas if a is less than 0.2, the reflectivity difference between the AlaGa1xe2x88x92aN layer and the GaN layer becomes small and it probably becomes difficult to obtain the multilayered reflective membrane with a sufficient reflectivity. The Al mixed crystal ratio a is more preferable to satisfy 0.3xe2x89xa6axe2x89xa60.7 and in such a case, it is made possible to obtain a remarkably high reflectivity difference and excellent crystallinity.
Further, in the above-described multilayered reflective membrane, xcex11 has preferably a value of not greater than 0.75. It is because if xcex11 exceeds 0.75, the film thickness of the AlaGa1xe2x88x92aN layer becomes too thick and the crystallinity deterioration of the multilayered reflective membrane probably becomes significant. More preferably, a satisfies the relation of xcex11xe2x89xa60.5 and in such a case, the film thickness of the AlaGa1xe2x88x92aN layer becomes sufficiently thin and the crystallinity of the multilayered reflective membrane becomes extremely excellent.
The multilayered reflective membrane as described above is suitable to be used for a gallium nitride-based light emitting element having an active layer of InxGa1xe2x88x92xN (0xe2x89xa6x less than 1). Hereinafter, the gallium nitride-based light emitting element of the present invention will be described. The gallium nitride-based light emitting element of the present invention has a multilayered reflective membrane formed on a substrate, said multilayered reflective membrane being deposited on at least one side of an active layer composed of InxGa1xe2x88x92xN (0xe2x89xa6x less than 1) through a nitride semiconductor layer.
Further, the multilayered reflective membrane may be formed between the active layer and the substrate. Furthermore, the light emitting element of the present invention may be formed by depositing an n-type clad layer, an active layer composed of InxGa1xe2x88x92xN (0xe2x89xa6x less than 1), and a p-type clad layer in the order and include the multilayered reflective membrane between the substrate and the active layer. Such a gallium nitride-based light emitting element has a multilayered reflective membrane with a high crystallinity and a high reflectivity between the substrate and the active layer, so that the crystallinity deterioration of the respective nitride semiconductor layers to be deposited on the multilayered reflective membrane can be suppressed and crack formation and occurrence of morphological abnormality can be prevented and the threshold current value and the threshold voltage value of the light emitting element can be lowered.
Further, the nitride semiconductor layer formed between the substrate and the active layer may be a superlattice layer. Since the superlattice layer has a low electric conductivity, the efficiency of carrier injection to the active layer can be improved, thereby further lowering the threshold current value and the threshold voltage value of the light emitting element. Furthermore, the superlattice layer may be formed so as to contact directly with the multilayered reflective membrane, and further the superlattice layer may be formed so as to contact directly with the active layer, thereby further improving the efficiency of carrier injection.
Further, such a gallium nitride-based light emitting element of the present invention is preferable to be employed for a surface emitting type laser element that emits light in the perpendicular direction to the main surface of a substrate.